Final answer:
The effective intervention for preventing or ameliorating smallpox after exposure is rapidly administering the smallpox vaccine.
Step-by-step explanation:
The intervention that is effective in preventing or ameliorating smallpox infection after exposure is D) Rapidly administering the smallpox vaccine. The smallpox vaccine, which originated from the practice of inoculation with cowpox lesions, introduces an immune response in the body that protects against the smallpox virus. This method paved the way for vaccination programs that not only provide individual immunity but also help in fostering herd immunity in populations. Vaccination as a preventive measure has been historically significant in the eradication of smallpox, demonstrating its efficacy in control and prevention of viral diseases.